1. Field of the Invention
This invention relates to hydraulic valve control circuits, and more particularly to valve operating circuits for providing positive opening and closing of downhole safety valves while preventing leakage of fuel to the outside environment.
2. Description of the Prior Art
Crude oil and gas wells are often drilled and tubing is installed at locations where the internal pressure of the petroleum deposit is quite high so that precautions must be taken to prevent a blowout of the well. Such blowouts are not only costly in terms of loss of oil or gas but in addition a blowout is highly dangerous and the cost of controlling a blowout at an oil or gas well is rather high. As a result, many devices including safety valves and associated control circuits have been developed and many such devices have been installed in association with gas and oil wells. One such device which is frequently employed is a surface-controlled, sub-surface, safety valve (SCSSV) otherwise known as a downhole safety valve (DHSV) which may be installed within the tubing of a well either at the time the tubing is installed or alternatively such a valve can be installed from the surface using well-known wire line techniques. Such valves are generally installed 200 or 300 feet below the wellhead and are always of the "fail-close" design. The construction of these valves resembles a conventional ball valve and positive actuation against a spring is required to open a valve, for example, by applying hydraulic pressure to a small diameter control line and to a valve actuator which can be conveniently located within the well. In some of the installations the valve actuator can be positioned outside the tubing.
The controlling hydraulic pressure applied to the control line must be sufficient to develop a force on one face of the piston of the actuator which is greater than the combination of the opposing force developed by gas or oil pressure in the tubing acting on the opposite face of the piston and by the spring-generated valve closing force. Because of the depth of the safety valves there is a substantial fluid head in the control line which provides a substantial amount of tubing pressure acting on the piston of the actuator, so that the spring force and the valve depth and the location of the safety valve must be carefully selected to ensure complete closure of the valve when the pressure in the control line is relieved by action taken at the surface.
Another type of SCSSV hydraulic circuit in common use involves a hydraulic balance and requires both a hydraulic control line to open and close the valve and a balance line which communicates with the opposing face of the piston of the actuator. By means of this arrangement, the control line pressure needs only to overcome the spring force since otherwise the forces are equal but opposite as developed by the head in both the control line and in the balance line.
Whether a balanced type SCSSV or a non-balanced type is used, it is common practice to pass the control and/or balance lines through the wellhead and its connector and then exit the christmas tree below the master valve. The control and/or balance lines, after leaving the christmas tree, are connected to a control system to enable operation of the SCSSV.
The previously proposed control systems have the disadvantage that if a malfunction such as a leak occurs in the DHSV, which results in connecting the tubing bore to the control line, a high pressure leakage path is then formed to the outside environment. Such a leak can damage the control system and also allow oil or gas to pollute the environment. This problem has already been appreciated and with a view to solving it, shut-off valves have been provided where the control and/or balance lines leave the christmas tree. By this provision, if a leak should occur, the shut-off valves can be closed manually but further problems arise if the christmas tree is installed below the surface of the sea because the shut-off valves will then require actuators, for example, hydraulic actuators so that the shut-off valves can be remotely opened or closed.
It will be apparent that the shut-off valves in the control and/or balance lines must be open when it is desired to open the associated DHSV or SCSSV so that fluid can be forced under pressure to the actuating cylinder of the DHSV or SCSSV. Even more important, the shut-off valves must remain open until the DHSV or SCSSV has completely closed. Once the latter has closed it is desirable to close fully the shut-off valves. However, if the shut-off valves are allowed to close before the DHSV or SCSSV has completely closed, the shut-off valves will not allow fluid to flow away from the actuator of the DHSV or SCSSV, and therefore the latter will remain open or partially open. It follows that for fully safe operation there must be proper co-operation between the actuator of the DHSV or SCSSV and the shut-off valves particularly for remote or sub-sea surface locations. In order more fully to take into account the difficulties outlined above, control systems such as hydraulic sequencing or electro-hydraulic multiplexing systems have been proposed so that the shut-off valves are connected to separate hydraulic output lines of the control system and are actuated independently of the DHSV or SCSSV control line. These proposed control systems are generally satisfactory but do not provide for the sudden loss of hydraulic pressure in the control system. Such loss in hydraulic pressure will result in the well becoming shut down because all the valves from the christmas tree including the DHSV or SCSSV will close because of their "fail-close" characteristics. However, the loss of hydraulic pressure will provide no assurance that the shut-off valves will remain open long enough to allow complete closure of the associated DHSV or SCSSV.
As an alternative to the complexities of hydraulic sequencing or electro-hydraulic multiplexing, a simple hydraulic time delay circuit has been proposed which comprises simply a restrictor valve and an accumulator which ensures that the DHSV or SCSSV closes before the shut-off valve is timed to close. This system has the merit of simplicity but does not provide a complete answer to the problems involved. In particular it is neither possible readily to know the exact closing time of the DHSV after installation nor is it possible to ensure that it will remain constant over long periods of time. To ensure that the system is basically safe, it has been proposed simply to make the time constant long enough to accommodate the longest possible closing times for the DHSV or SCSSV. However, such long time constants require either very small orifice restrictor valves which are liable to clog or large accumulators which cannot readily be accommodated in the limited space available.